In the DefaultKeyboardFocusManager class we have a special field "activeWindow", which stores the currently active window. It is used in two similar cases:
1. If the java window gets "WINDOW_ACTIVATED" event it will try to send "WINDOW_DEACTIVATED" to the old active window, which is stored in the "activeWindow" field.
2. If the java component lost the focus, and the opposite component is not a java component, then it will try to send "WINDOW_DEACTIVATED" to the old active window, which is stored in the "activeWindow" field.
The difference in these two cases is that in "case 1" we check the old active window to null[1], and the second case has no such check. The bug is reproduced in non-standalone mode, when we have a few Appcontexts and this field might be updated by different EDT in parallel.
Note that the test is for OSX only, because of another bug: JDK-8204142[2]
[1] http://hg.openjdk.java.net/jdk/jdk/file/ad9077f044be/src/java.desktop/share/classes/java/awt/DefaultKeyboardFocusManager.java#l527
[2] https://bugs.openjdk.java.net/browse/JDK-8204142